Transferring liquids such as chemicals, effluents, or molten metals by using multi-phase flow technology is known in the art. Practically everything and every conceivable concept, as well as all the related theories for their design, reaction, modeling, gas absorption, heat transfer, etc., has been covered with infinitesimal detail in the book by Wolf Dieter Deckwer, first published under the title Reacktiontechnik in Blasensaulen, Copyright 1985, Otto Salle Verlag GmbH & Co., Frankfurt am Main, Verlag Sauerlander AG, Aarau, Switzerland. Additional studies have been conducted by Frede Frisvold, Thorvald A. Engh, and Didrik S. Voss as early as 1985.
The earliest systematic investigation of a multi-phase (gas/liquid) pump began in 1968, by Lu Hongqi and Liang Zhongtian, Wuuhan Institute of Hydraulics and Engineering, Peoples Republic of China, who have through the years proposed the basic theoretical equations and boundary condition equations that govern two-phase flow utilizing flow models. Using the extensive knowledge available, some designs have been proposed to pump molten metals. Among them, Alphatech/Alcoa, tested bubbling gas (nitrogen) inside tubes to generate metal motion, and analyzed the mixing of nitrogen in the liquid metal for the purpose of removing hydrogen entrapped in the liquid metal as early as August of 1990.
Later, Larry D. Areaux and Brian Klenoski were issued U.S. Pat. No. 5,203,910, Apr. 20, 1993, in which the vertical column suggested by Wolf Dieter Deckwer was replaced by an inclined column to effect the recirculation. See FIGS. 17 and 18.
In plants where aluminum scrap is melted converting the metal to liquid aluminum and then to cast products, it is customary to prepare alloys in batches of 50 tons or more. The composition and temperature of the liquid metal must be closely controlled. Predictable metal temperature means predictable timing and it becomes possible to schedule a greater output with less capital expenditure. These furnaces are fired with natural gas or fuel oil.
The inventive equipment obtains temperature and alloy homogeneity in the furnace, and provides a method for stirring the liquid metal to equalize the temperature in the furnace, and eliminating the thermal gradients in the liquid metal to optimize the alloying elements dissolution rate. The preferred method removes undesirable gasses entrapped in the aluminum melt by impinging inert gas at high velocity during the recirculation process. A method is disclosed for manufacturing this equipment to maximize its reliability, integrity, and life to withstand the rigorous environment and treatment to which it is subjected. Further, a method is disclosed for recovering the inert gas from the equipment, in order to minimize additional expense.
As the density of aluminum decreases with increasing temperature, the application of heat over the metal pool in the furnace produces a transient thermal gradient. When the pool depth in the furnace is approximately 36" and the pool is heated from above, approximately 30 minutes elapses before that heat reaches the bottom of the furnace. Because of aluminum's high reflectivity there is very low observable liquid metal convection. The heating rate of 50 tons of metal is in the vicinity of 106.degree. F. in half an hour. Therefore the bottom temperature lags by half an hour. Gradients develop which approach 200.degree. to 250.degree. from the top to the bottom of the melt.
To overcome the temperature control problem, to reduce energy consumption and to improve the reliability of alloying, forced stirring, or metal recirculation, of the melt is necessary. Electromagnetic and mechanical means are possible.
Electromagnetic means is ruled out because of the incredible installation costs. Mechanical means require a pump well outside the furnace proper, which further cools the molten metal, and introduces additional energy loss. The mechanical pumps currently used are subject to continuous failures and very high maintenance costs because of the severe environment. The inventive pump can be introduced into such a furnace below the metal line to effectively mix large tonnages of liquid metal while firing the furnace, thus permitting good temperature control, and fuel and time economy.
If a continuous jet of liquid is injected into a body of that liquid, then Fox and Gex (A.l.C.H.E. Journal 2.4.1956. Pg. 539) have shown that the mixing time of the body is given by: ##EQU1## Y=depth of the body of liquid D.sub.t =diameter
N.sub.re =Reynolds number of the liquid PA1 U=Kinematic viscosity of the liquid PA1 G=gravitational acceleration PA1 V.sub.o =jet velocity PA1 D.sub.o =jet diameter PA1 P2=absolute outlet pressure (usually .about.18.3 PSIA); and PA1 P1=absolute inlet pressure PA1 1. The flow between the gas jet and the suction of liquid metal is relative in motion, in which the liquid metal is sucked by the gas jet boundary with a transfer of momentum from the gas to the liquid. At this stage, the liquid and the gas are considered separate mediums. PA1 2. Under the action of the boundary gas jet velocity, the gas is broken into very small bubbles that are distributed in the liquid. As the bubbles impact the liquid molecules, the gas is compressed in the convergent zone of the nozzle and dispersed in the liquid. PA1 3. The gas bubbles are surrounded by liquid drops. The liquid drops coalesce into a mixture with the bubbles trapped in it, carried forward and further compressed. In this stage the liquid is considered the continual medium and the gas is distributed in the liquid as bubbles.
When the properties of the tank and the fluid are constant, ##EQU2## where N is the number of molten metal jets used.
A single jet pump inserted into a bath of aluminum inside the furnace has, (see FIGS. 23 and 24) when providing suitable mixing, the advantage of extreme simplicity (no moving parts immersed in the liquid aluminum).
The problems with prior art devices which move molten metal in a bath using two-phase flow technology is that the designs use bubble-lifting technology, which is extremely slow, has very poor effective gas distribution, poor gas dispersion in the metal and low flow velocity. The Areaux et al. design is aggravated by the inclined tube configuration. The operating efficiency and maximum velocity of a bubble pump reactor is obtained when the tube is vertical, since the head lifting capacity of the pump is dictated by the height of the molten metal pool. The bubble has to travel a longer distance in an inclined tube, thus increasing the time to reach the surface, and, consequently, reducing the velocity of the metal flow and the efficiency of the pump. It is also obvious by examining the Fox Gex equation that the velocity of the liquid aluminum stream inserted into the aluminum melt as well as the cross-sectional area of the stream should be as large as possible, since the time required to equalize the temperatures is inversely proportional to these two factors. Obviously, bubble column pumps do not have these attributes.
Another detrimental characteristic of the Areaux et al. bubble design is that the nitrogen gas is injected in the inclined tube perpendicular to the direction of metal flow. This is necessary to avoid additional severe complications in the design and manufacture of the inclined tube pump. Because of this, the injected gas acts as a fluidic restrictor, or shut-off valve (see FIG. 18) that prevents the metal from either flowing in the direction of the tube or entering the tube since the gas injected at the bottom of the tube is trying to expand in both directions.
An additional detrimental characteristic of the inclined tube bubble pump is that it forms elongated bubbles because they are trying to expand vertically toward the surface faster than toward the inclined outlet of the tube, thus creating a large back-flow of metal that reduces the pump efficiency to ranges well below 20%, (see FIG. 17). In addition, to allow the necessary time to generate a large enough bubble to seal the inclined tube and to keep the gas from impinging against the opposite wall of the tube and creating severe material damage because of the cavitation and erosion effect created, the inlet pressures that can be applied must be maintained far below sonic ratios.
Tests conducted by the writer on a typical inclined tube bubble pump of 21/2" diameter and a 45.degree. angle show that the inlet pressure could not be below a P2/P1=0.83, where:
At P2/P1 ratios below 0.83, the gas started exiting toward the lower end of the tube, stopping all possible flow for tubes inclined to a 45.degree. angle (see FIGS. 17 and 18). In other words, the gas inlet pressure for most furnace applications could not exceed 22.0 PSIA (7.3 PSIG). To achieve gas sonic velocity in a nitrogen gas flow process (K=1.4), the ratio P2/P1 must be maintained below 0.528 which will require a gas inlet pressure of 34.65 PSIA (19.95 PSIG) minimum, almost three times the maximum of an inclined tube bubble pump. This is not improved by pulsating the gas input since the average velocity of the gas and the metal remain almost unchanged and extremely slow. In tests conducted, the maximum metal flow velocity obtained was 12 to 14 in/sec, while the minimum required for a proper recirculation/degassing unit should be no less than 40 in/sec. A standard motor-driven recirculation pump has a metal flow velocity of approximately 40 to 60 in/sec. Based on the available test data, it can be stated that the maximum gas flow velocity in an inclined tube bubble pump will be approximately 112 ft/sec. The sonic flow velocity of nitrogen under the conditions stated (aluminum temperature 1740.degree. R., P2=18.3 PSIA), ##EQU3##
This is 5 times the maximum inlet velocity achievable on an inclined tube bubble pump with radial gas injection. Obviously, Areaux et al. have been extremely optimistic in the assessment of the performance of their pump.
Therefore, the bubble pump design is not an efficient recirculator degasser or dross emulsifier because effective recirculation velocity, degassing and dross emulsifying is only obtained by injecting the gas into the molten metal at the highest possible velocity (sonic or nearly sonic), in order to obtain the maximum possible metal flow velocity and gas dispersion into the metal for optimum removal of the entrapped gasses. When a high level of gas dispersion and flow velocity is the end result of forced liquid recirculation, the utilization of gas jets oriented centrally and axially in the direction of the metal flow is absolutely mandatory. The pumping of metal by the slow formation of large bubbles does not provide any of the basic stated requirements.
The design of multiple central axial jet gas injection distribution with an elliptical cross-section in the metal-lifting conduit, as shown in FIGS. 5 and 6, was disclosed in my patent application Ser. No. 08/560,661, filed Nov. 20, 1995, for a jet bubble-operated recirculating pump for a metal bath.
Because of the inclined tube's configuration, the Areaux et al. multiple porting gas injection does not work because it aggravates the fluidic shut-off valve effect. In my design (see FIGS. 5 and 6), the power jets create a high energy dissipation zone in which the gas is broken up into very small primary bubbles. The bubbles then coalesce to form large bubbles. An equilibrium bubble diameter is established that remains the same throughout the remainder of the conduit.
The extent of the coalescence and size of the bubbles at the equilibrium zone depends on the number of nozzles, the inlet and outlet pressures, the head of metal above the gas injection point and the liquid metal properties. Although the design in FIGS. 5 and 6 already presents great advantages with respect to efficiency, flow velocity and gas dispersion over that of an inclined tube design, testing and analyses conducted by the applicant confirm that additional compression of the gas into the liquid metal is required to achieve true degassing and high flow velocities that are not totally dependent on the liquid metal head above the pump.
Based on these evaluations, the pump configuration shown in FIGS. 2 and 3 has been created. A convergent/divergent nozzle zone feature has been added to the pump's vertical section, since in a jet column reactor the metal flow velocity and gas dispersion are not a function of the metal head above the pump. This assures, by accelerating the metal at the throat section of the tube nozzle, that a faster intermixing and a forcing of the gas dispersion into the metal will take place, retarding the gas coalescence and tendency to aggregate too soon into larger bubbles. The metal conduit nozzle area to throat area ratio is the most important design element for jet pumps and serves as a criterion in the same manner as specific speed does for centrifugal pumps (J. J. Whitte "Efficiency and Design of Liquid Gas Ejectors", British Chemical Engineering, Vol. 9, September 1965). Theoretical studies performed by Lu Hongqi indicate that this type of pump, when properly designed, should provide a higher velocity at a given flow than any centrifugal pump. With an output head 50% higher than that of a centrifugal pump, this translates into a proportional increase in outlet velocity. ##EQU4## This steep head capacity characteristic was corroborated in water testing by R. G. Cunningham, (Gas Compression with a Liquid Jet Pump, Journal of Fluids Engineering Transactions, A.S.M.E., Serial 1,96,3, September 1974). As there is a true two-phase flow, a unit weight of the liquid (molten metal+gas) is very different from that of the gas and that of the molten metal. The evaluation of the flow pattern is highly complex. The performance of what I call the "jet column degassing and dross diluting reactor" is related to the type of the conduit structure ("S", "C", "L", "T" and "U" shapes in this patent application, see FIGS. 2-5 and 19-21), number of gas injecting nozzles, inlet/outlet pressure ratio and physical orientation.
Another great difference exists between my inventive design and standard bubble column pumps because my pump will operate in any position (from horizontal to vertical) and generate flow upwards or downwards without a loss of efficiency, (see FIGS. 19-21), since it utilizes the energy transfer from the gas to the liquid, acting as a flow transfer machine and mixing reactor. Bubble pumps only flow upwards (inclined or vertically), and their efficiency is a function of the angle of inclination. Bubble pump designs only utilize the energy provided by the head of metal above the point of gas injection. If the column in a bubble pump, instead of being inclined, is in a horizontal position, the output and efficiency of the bubble pump would be zero (.DELTA.H=0). The transfer of energy in my pump, from the gas and its momentum to the liquid metal, is effected by the convergent/divergent nozzle provided on the straight portion of the "S" or "C" shapes, or the horizontal section shown in the "T" and "U" configuration (see FIGS. 2, 3, 5, 8, 19 and 21).
The general description of the operation of my inventive pump, as shown in FIGS. 2 and 3, can be broken into the following stages:
There has been a stage of semi-experiment and semi-theory in the study of liquid/gas jet pumps, mostly where the element injected at sonic velocity is the liquid, and the gas is provided for the purpose of dispersion because of its flammable, explosive or radiation condition. In my inventive pump, the liquid is in a metal pool, and the gas media is injected at near sonic or sonic velocity through the use of multiple nozzles centrally and coaxially aligned with the straight section of the "S" or "C" shaped conduit. Some of the formulations obtained by Lu Hongqui (the equations and critical flow conditions) have been used to size the experimental pumps. Verification of liquid metal flow and degassing efficiency were performed in both water and molten metal (aluminum), starting in November of 1994. For additional views of the "S" shaped and "C" shaped configurations, refer to FIGS. 3-8, 25 and 26.
My inventive pump also addresses the breakage and erosion problems encountered with pumps moving molten metals for recirculation or degassing purposes. A pump made of a relatively thin-walled ceramic material has been disclosed in my U.S. patent application Ser. No. 08/560,661, filed Nov. 20, 1995, for a bubble-operated recirculation pump for a metal bath. The problem with a thin-walled ceramic device is that, although it is extremely resistant to erosion and corrosion from either the liquid metal or the dross in the metallic bath, the device is brittle and generally breaks when mistreated by the furnace operators. For example, when the furnace metal pool is loaded with solid metal ingots, the impact from one of these ingots can permanently damage a relatively fragile pump.
My improved pump encases the basic pumping conduit in a refractory body (see FIGS. 9 and 10). A ceramic conduit is placed in a box or mold and encased in a refractory mix after which it is fired dry in a kiln. Both the nitrogen feeding conduits and the thin-walled lifting conduits are then firmly encased in refractory material, thereby eliminating the possibility of breakage of the ceramic material. Tests conducted with this configuration show excellent life and impact resistance.
The preferred embodiment of my invention can also be made with a refractory body, without the use of a liner, by the well-known lost-wax method or other similar methods, where the pattern core is dissolved or melted. A device having no liner is especially useful in a zinc bath. The refractory material is basically a combination of alumina and silica and extremely resistant to molten zinc or zinc/aluminum alloys where the percentage of aluminum is below 25%. On the other hand, in an aluminum bath, aluminum is known to attack the silica material by alloying itself with the silicon in it and releasing the oxygen, forming dross that clogs the lifting conduit. For these particularly high aluminum alloys or aluminum applications, the refractory should be silica-free alumina.
A monolithic casting with a ceramic liner is not only extremely inert to aluminum attack up to temperatures in the order of 2000.degree. F.; but, in addition, it is very durable, hard and abrasion resistant to impurities carried by the molten metal. It can withstand severe cavitation problems that could be created by an improper lifting conduit configuration (inclined tubes with sharp turning corners as depicted in the Areaux et al. bubble pump patent (see FIG. 17), where a sharp transition from the inclined to the horizontal is prone to create severe cavitation damage in the tube, be it ceramic or any other material).
An additional advantage of my inventive reactor pump is that by utilizing my monolithic jet column degassing and dross diluting reactor, the conventional outside pumping well of recycling furnaces can be eliminated by recirculating the metal inside the furnace bath by installing a "C" shaped configuration jet column reactor in each corner of the furnace (see FIG. 15). The scrap can be loaded in the recycling furnace directly through a funnel conduit, minimizing heat loss and maximizing energy efficiency. The outside well needed for installation of the recirculation and degassing pump is eliminated (see FIG. 16).
Another application and advantage of the "C" shaped jet column reactor is that in the zinc and aluminum baths in the galvanizing industry, the dross comprising iron, aluminum and zinc/aluminum sinks to the bottom of the pot. This dross accumulates to the point where it touches the sink roll, around which the strip being galvanized is passing, thereby contaminating the strip and, on some occasions, completely stopping the rotation of the roll.
The advantage of my monolithic pump configuration is that, when placed at the bottom of the pot, it can be used to continuously recirculate the bottom dross. The jet gas disperses it into the liquid metal to prevent build-up. Preferably the bottom of the galvanizing pot is formed with a low spot, so the bottom dross will tend to concentrate at a location where it can be easily sucked in through the bottom inlet of my jet column reactor.
Yet another advantage of the jet column reactor is that the metal, gas flow velocity and gas dispersion capacity is not a function of the metal head above the pump. By increasing the pressure ratio between inlet and outlet to sonic (P2/P1&lt;0.528), dross that has already been crystallized will become emulsified and its density reduced, generating a tendency for it to float. The floating dross can then be easily skimmed off the bath (see FIG. 19 and 26).
The preferred device, as shown in the drawings, uses a multi-orifice/nozzle (nitrogen, argon or helium feed) arrangement. Several small orifices are necessary and advantageous over a single large orifice because a very small high velocity jet generates bubbles which expand very fast past the nozzle throat, due to surface tension and the differential pressure between the gas and the metal. As the bubbles increase in diameter, they expand slower, reducing the total area exposed to contact with the metal and reducing the degassing ability of the pump (Sigworth G. K., 1982, "Hydrogen Removal from Aluminum", Meeting Trans. B, vol. 13B, pp 447-460).